The present invention concerns filter media in general. In particular, the invention concerns those filter media that are provided with a nanofiber coating. Moreover, the invention concerns a method for producing such filter media.
For the purpose of air filtration, it is advantageous when, for the filtration task, electrical charges are permanently available for a targeted separation of particles by electrostatic interaction (so-called electret effect). Such electrical charges on the surface of an air filter medium can increase the efficiency of the particle separation; however, they dissipate very quickly, in particular in the presence of high air humidity and high temperatures. In order to solve this problem, a known approach is based on using charge-stabilizing additives. Problematic in this context is however that the additives are effective only within very narrow limits, for example, only within a very narrow concentration and processing window (for example, cooling rate of the polymer melt during processing) of the additives, as described by N. Mohmeyer et al., “Additives to improve the electret properties of isotactic polypropylene”, Polymer, 2007, vol. 48, pages 1612-1619, Elsevier Ltd. Also, the additive effect is often limited only to a certain polymer matrix and a certain kind of charging.
A further known approach is based on so-called triboelectric media. For this purpose, fiber mixtures of polypropylene (PP) and polyvinylchloride (PVC) or polypropylene (PP) and polyacrylonitrile (PAN) are used. For example, U.S. Pat. No. 6,623,548 B1 discloses an electrostatically charged filter material that comprises a mixture of PP fibers and fibers selected from the group consisting of halogen-free acrylic fibers, PVC fibers or a mixture of halogen-free acrylic fibers and PVC fibers. Even though in this context the charges dissipate also by the action of air humidity and temperature, these media however always automatically recharge themselves by friction (caused by the air flow). A disadvantage is that the use of PVC in the automotive industry is no longer desired and the prices for PAN are relatively high because it is also used for manufacturing carbon fibers.
It is known that closed cavity polymeric structures (polymer foams) can be charged electrically. In this context, by applying external electric fields (for example, corona discharge), electrical breakdowns are generated in the closed cavities. These effects have been described, for example, by G. S. Neugeschwandtner et al., “Piezo- and pyroelectricity of the polymer-foam space-charge electret”, J. of Appl. Physics, 2001, vol. 89 (8), pp. 4503-4511. The cavities can be produced, for example, by saturation of a polymer film with a low-boiling liquid and subsequent sudden evaporation of the liquid (compare N. Behrendt et al.; “Piezoelectricity of Cellular Polypropylene Films Expanded by a Dichloromethane Evaporation Process”, Advanced Engineering Materials, 2008, vol. 10, no. 1-2, pp. 120-125).
U.S. Pat. No. 6,520,425 B1 discloses the so-called electroblowing method in which air as a “shaping” medium is flowing about a nanofiber-generating nozzle in order to generate particularly thin fibers. It is however not possible to generate cavities in the thus produced nanofibers by means of this method.
There is therefore a need to make available a filter medium with quasi-permanent charges for filtration.